This invention relates to inducing compression along and into the surface of a component and more particularly to a method and apparatus for inducing compressive residual stress along and into the surface of a workpiece having a regular or an irregular surface topography.
Many metallic machines and various structural components are subject to failure by fatigue, corrosion fatigue, or stress corrosion cracking (SCC). Failures generally initiate from the surface of the component in highly stressed areas, often from scratches, corrosion pits, or other surface damage that creates a shallow notch or indentation that produces a local stress concentration. It is well known that surface enhancement, such as the introduction of a layer of compressive residual stress, can if of sufficient magnitude and depth, mitigate the stress concentration due to the damage and greatly improve the “damage tolerance” or fatigue strength and service life of a component. Further, since SCC requires surface tension above a threshold level, placing the surface in residual compression can eliminate or significantly reduce SCC.
The introduction of compressive residual stress is achieved in all practical cases by introducing non-uniform cold work, or plastic strain, into the workpiece. The resulting amount and distribution of residual stress and the resulting change in shape depend upon both amount and distribution of plastic strain and the original geometry of the workpiece.
The introduction of residual stress is also used in the forming of components, such as the curved skin of aircraft wings. For forming applications the magnitude, depth and distribution of the induced stress throughout the workpiece are critical properties requiring precise control.
In some applications, the cold working of the metallic material is used to modify the mechanical and chemical properties of the existing surface or a surface layer deposited by plasma spray, cold spray, plating, or some other process. The original surface and/or the deposited surface layer is deliberately cold worked to a required amount to achieve the desired properties, such as work hardening. Cold working may be followed by heat treatment for crystalline grain refinement or to promote diffusion and bonding of a coating to a substrate. For these surface modification applications the magnitude and distribution of cold working are the critical processing properties that require precise control.
A variety of surface enhancement processes have been developed. Hammer peening of welds is an ancient practice known to eliminate residual tension caused by shrinkage of the hot weld, but is an uncontrolled manual process. Modern processes for inducing compression along and into the surface of a workpiece include shot peening (SP), laser shock peening (LSP), low plasticity burnishing (LPB), deep rolling (DR), ultrasonic peening (UP), ultrasonic needle peening (UNP), flapper peening (FP), and cavitation peening (CP). However, all such methods have limitations that make automated application to certain surfaces, such as irregular topography surfaces often found in welded assemblies, difficult or undesirable.
Application of LPB, DR, CP and LSP all require that the surface of the workpiece or component, to be processed be well defined geometrically so that the mechanical burnishing tool, the cavitation zone, or the laser focal spot can be accurately positioned during processing. The positioning requirements for these methods are similar to machining. Automated processing of welds or other irregular topography surfaces using CNC control is difficult because the workpiece shapes, surface geometries, or irregularities vary making the process non repeatable. Therefore, components having irregular topography surfaces, such as manually welded assemblies, cannot be reliably treated because the irregularities may cause the processing tool to be positioned too close or distant from the surface to be effective, and some regions may be missed altogether during processing.
SP, UP and UNP all utilize a blast of shot propelled from nozzles or thrown from a wheel, a fluidized cloud of shot ricocheting in a chamber, or clusters of randomly impacting needles to deform the surface by covering it with dimples. Programmed robotic direction of shot flow from nozzle peening systems is a common practice. FP utilizes a rotating flexible sheet studded with impacting media (shot) generally positioned manually. One such flapper peening system is disclosed in U.S. Pat. No. 7,954,348 that controls the speed of the rotating “flapper” to regulate the impacting force and speed. While these methods can accommodate processing of an irregular topography surface, such as a manual weld, they impact the surface randomly thereby making it difficult to achieve the optimum surface processing necessary for certain applications. Further, to achieve full coverage of the treated surface the media (shot or needles) impact the surface repeatedly, often as many as 16 times on some areas in order to be sure that most of the surface has been impacted once. The repeated impacts can highly cold work the surface which can be detrimental to work hardening alloys, leaving a compressive layer that is subject to rapid thermal stress relaxation or mechanical overload relaxation in service. Cold working also work softens hardened steels leaving a softened surface layer, and transforms retained austenite causing slight swelling and often results in an unacceptable change in critical dimensions. The depth of compression achievable by shot is limited by the size of the media used, and is generally more shallow than the depth of compression induced by LPB or LSP. Finally, the repeated impacting required of these methods is also simply inefficient in terms of energy usage.
Robotically controlled hammer peening has been developed such as for the peening of welds, where the impacting head follows a fixed path defined by the robot control code. However, such systems do not provide an effective method of controlling and monitoring the performance of the peening process, or for accommodating irregularities in the surface of the workpiece thereby reducing or eliminating the likelihood of inducing the desired or effective compression along surfaces having such irregularities.
Accordingly, a need exists for a method and apparatus of inducing compressive residual stress along and into the surface of workpiece; that can be automated, such as by robotic or CNC machine tools; produces a controlled desired depth and magnitude of compression and cold work; and can be reliably applied with process monitoring to workpieces having an irregular topography surface.